A general frame structure for a wireless access system will be described below.
FIG. 1 illustrates a frame structure in a broadband wireless access system (e.g. Institute of Electrical and Electronic Engineers (IEEE) 802.16).
Referring to FIG. 1, in a frame, the horizontal axis represents Orthogonal Frequency Division Multiple Access (OFDM) symbols as time units, and the vertical axis represents the logical numbers of subchannels as frequency units. In FIG. 1, the frame is divided into data sequence channels each having a predetermined time period according to physical characteristics of the frame. That is, a frame includes a DownLink (DL) subframe and an UpLink (UL) subframe.
The DL subframe may include a preamble, a Frame Control Header (FCH), a DL-MAP, a UL-MAP, and one or more data bursts. The UL subframe may include one or more data bursts and a ranging subchannel.
In FIG. 1, the preamble is predetermined sequence data in the first symbol of every frame. With the preamble, a Mobile Station (MS) acquires synchronization to a Base Station (BS) or performs channel estimation. The FCH carries DL-MAP-related channel allocation information and channel code information. The DL-MAP and the UL-MAP are Medium Access Control (MAC) messages that carry DL and UL channel resource allocation information to MSs. The data bursts may be used for units of data directed from a BS to MSs or from MSs to a BS.
A Downlink Channel Descriptor (DCD) that can be used in FIG. 1 is a MAC message describing physical characteristics of a DL channel and an Uplink Channel Descriptor (UCD) that can be used in FIG. 1 is a MAC message describing physical characteristics of a UL channel.
Referring to FIG. 1, on the downlink, an MS detects the preamble transmitted from a BS and acquires synchronization to the BS using the preamble. Then the MS can decode the DL-MAP based on information acquired from the FCH. The BS can transmit scheduling information for DL or UL resource allocation to the MS every frame (e.g. every 5 ms) in the DL-MAP/UL-MAP message.
The DL-MAP/UL-MAP message structure illustrated in FIG. 1 may cause unnecessary MAP message overhead because the BS transmits the MAP messages at a Modulation and Coding Scheme (MCS) level that allows all MSs to receive the MAP messages commonly irrespective of their channel statuses.
For instance, MSs near to the BS are in good channel status and thus the BS may use a high MCS level (e.g. Quadrature Phase Shift Keying (QPSK) 1/2) for message encoding and decoding for the nearby MSs. Nonetheless, the BS encodes the MAP messages at a low MCS level (e.g. QPSK 1/12) for MSs at a cell boundary. Therefore, each MS should receive the MAP messages encoded at the same MCS level irrespective of its channel status. As a consequence, unnecessary MAP message overhead may be created.
A resource allocation unit may vary with wireless access systems. For example, resources are allocated every 5-ms frame in an IEEE 802.16e system and every 1-ms Transmit Time Interval (TTI) in a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system. A MAP exists in every resource allocation unit, for radio resource allocation. In this context, a dedicated MAP is required for each MS in order to increase frequency efficiency and reduce the complexity of the MS.
3GPP LTE defines such a MAP message as a Downlink Control Indicator (DCI) and transmits the DCI on a Physical Downlink Control Channel (PDCCH) in the physical layer. There is a DL channel for delivering an ACKnowledgment/Negative ACKnowledgment (ACK/NACK) for an UpLink Shared Channel (UL-SCH). The DCI can be transmitted on a Physical Hybrid-ARQ Indicator Channel (PHICH) in 3GPP LTE.
FIG. 2 illustrates an exemplary subframe structure in the 3GPP LTE system.
Referring to FIG. 2, the positions of allocated Control Channel Elements (CCEs) and the position of a Reference Signal (RS) allocated for each antenna, for channel estimation are marked in a Resource Block RB). In the illustrated case of FIG. 2, a bandwidth of 1.25 MHz is used.
In a wireless access system (e.g. 3GPP LTE) system, a plurality of CCEs can be transmitted in first n OFDMA symbols of each subframe. A CCE may refer to a control information transmission unit. One CCE can be disposed in successive or distributed time-frequency areas.
One subframe has 14 OFDM symbols in the 3GPP LTE system. The first to three ones of the 14 OFDM symbols can be used for transmitting a Physical CFI Channel (PCFICH), a PDCCH, and a PHICH. This amounts to an overhead of about 7.1% (in case of one symbol) to 21.4% (in case of three symbols).
In FIG. 2, a Resource Unit (RU) is a basic allocation unit defined by 12 subcarriers and 14 symbols. The first one to three OFDMA symbols of an RB are occupied for control channels. Each control channel is composed of 4×1 basic units called mini Channel Elements (CEs).
The first symbol carries a PCFICH for transmitting a Control Frame Indicator (CFI). The CFI describes the number of symbols used for a control channel, occupying a total of four mini CEs. The first symbol also carries a PHICH for transmitting a Hybrid Automatic Repeat reQuest (HARQ) ACK/NACK (e.g. A/N mini CEs) for UL data. A PDCCH is delivered in the remaining control channel area. The PDCCH is allocated in units of CCEs. Each CCE may have nine mini CEs. To achieve frequency diversity, the CCE has mini CEs at different positions along the frequency axis.
In the 3GPP LTE system, a PDCCH allocated to each MS can be detected by blind detection. However, the blind detection is complex because it should be repeated tens of times (e.g. 40 to 50 times) depending on the total number of MAPs. Moreover, since as much blind decoding is required, the complexity increases considerably.
Aside from allocation of radio resources (e.g. a control channel) in symbols to a frame, there is a method for allocating radio sources in a plurality of subchannels along the frequency axis. The symbol-based control channel allocation is referred to as Time Division Multiplexing (TDM) and the subchannel-based control channel allocation is referred to as Frequency Division Multiplexing (FDM).
Despite the advantage of allocating radio resources to control channels at various ratios, the frequency-based radio resource allocation method allows data channel decoding only after control channel decoding, thus causing a time delay. The time delay may bring about one-subframe Round Trip Time (RTT) at worst in a system using subframes. Especially a Time Division Duplex (TDD) system may suffer from an about one-frame time delay (e.g. 5 ms in IEEE 802.16e).
If a submap occupies a whole OFDM symbol as done in a general TDM scheme, a DL subframe without a UL submap results in a great waste of unused subchannels.
In the case of scheduling for persistent control or Voice over Internet Protocol (VoIP), the use of submaps may further be reduced. Therefore, allocation of one entire OFDM symbol for a submap in every subframe leads to serious resource consumption.